1. Field of the Invention
The invention relates to an adjustable hydrostatic drive, particularly for auxiliary units of internal combustion engines of motor vehicles, comprising a pump and motor arranged radially one inside the other in a common housing (hereinafter referred to as a radially nested pump/motor unit), with an eccentric control ring between them, the pump and motor being hydraulically connected together and their delivery volumes being adjustable in opposite senses.
2. Description of the Prior Art
Internal combustion engines, particularly the driving motors of motor vehicles, have to provide auxiliary drives for units that serve either for the operation of the internal combustion engine itself or other purposes. Examples of units serving for the operation of the internal combustion engine itself are water pumps, fans, generators and lubricating oil pumps. For other purposes there are, for example, oil pumps for power-assisted steering, level control and air suspensions. The performance of the units has to be adequate even when the engine is idling. Some of these units always require their full, speed dependent power, while others, for example a generator and a fan with a viscous coupling, take off a controlled amount of power, and yet other units are occasionally operated at their normal power and are then switched to idling power; examples of these are compressors for air conditioning units and air suspensions.
It is known for all the units to be switched on at their maximum power. The sum of the individual powers can then reach a value such that the idling power needed from the internal combustion engine can only be provided by increasing the idling speed. An increased idling speed is however undesirable, since it increases the noise generated; when hydrodynamic converters or clutches are used the creep moment and heat of slippage are increased; and the total revolutions are increased.
So that the auxiliary units can be better served in the lower engine speed range without the need to accept excessive losses at high speeds, hydrostatic drives and hydraulic converters are known that comprise a pump and a motor. With these drives and converters the revolutions needed for the auxiliary units can be obtained by changing the delivery volume of the motor driving the auxiliary units relative to that of the pump that is rotating at the speed of the engine.
In a hydrostatic drive known from German Offenlegungsschrift No. 19 62 613 the pump and motor are in the form of vane-cell machines and are inserted radially one inside the other. Between the outer vane-cell machine that serves as the motor and the inner vane-cell machine that serves as the pump is arranged an eccentric control ring, the outer side of which cooperates with the vanes of the motor and the inner side with the vanes of the pump. According to the position in which this eccentric control ring common to the motor and the pump is set, the through-flow of fluid driving the rotor of the motor, and thus also the volume delivered by the vanes of the pump, can be altered in opposite senses, i.e. the volume delivered by the pump decreases while the volume delivered by the motor simultaneously increases, and vice versa. It is however not disclosed how the position of the control ring is changed and fixed so as to produce a variation in the revolutions.
With a wide speed range, for example a ratio of the idling to maximum revolutions of 1:10, such as occurs with a target idling speed of 600 to 650 r.p.m. and a maximum speed of 7000 r.p.m., the eccentricity needed is correspondingly large, i.e. part of the wall of the control ring must be very thick. Since the wall of the control ring is so thick, the volume occupied by the known hydrostatic drive is undesirably large, despite the radial construction.
A radially nested pump/motor unit comparable with the hydrostatic drive described above, and having the same disadvantages, is also known from U.S. Pat. No. 2 434 546.